1. Field of the Invention
The present invention relates to a digital TV, and more particularly, to a speed-change playback apparatus of a digital TV which performs better fast forward and backward performances by analyzing a transport stream inputted at the time of a time shift service and a method thereof.
2. Description of the Background Art
A time shift performance which is one of additional performances of a broadcasting receiver of a digital television is used when a user wants to review a previous screen which is missed while the user quits his place for a while by a slow motion and etc.
When the time shift performance is set as an enable state by the user, the digital TV receiver records a transport stream (hereinafter, will be called as a TP packet) which is currently received at a storage unit and a scene of the enable time point is displayed on a screen of the digital TV receiver as a stop state. Then, when the user presses a play button, scenes from the stop state to the stored state in the storage unit corresponding to the TP packet are displayed on the screen, so that the user can watch consecutive scenes without stopping even if he quitted his place for a while.
At this time, the digital television receiver can operate performances such as a rewind, a forward, or a fast forward by the user. Also, the rewind, the fast rewind, the forward, and the fast forward performances are possible from a time point when the time shift performance is started to a time point of a current broadcasting. In case that a scene is displayed to the current broadcasting time point by the fast forward performance, the digital television receiver turns off the time shift performance automatically and displays the current broadcasting scene on the screen.
In the conventional digital television receiver for the time shift performance, the fast forward, the fast backward, and the backward performances are realized by feeding the TP stream stored in the storage unit to an MPEG decoder IC in a forward direction or in a backward direction.
That is, in case of the fast forward of the forward direction, the digital TV receiver feeds the TP packet stored in the storage unit to the MPEG decoder IC with a higher rate.
At this time, the digital TV receiver ignores the TP packet, that is, timestamp information of the MPEG stream, and playbacks the TP stream on the screen with a maximum operation speed of the MPEG decoder IC. Herein, the playback speed of the MPEG decoder IC is two times or four times and so on.
Generally, the TP packet includes an intra frame (hereinafter, will be called as I picture) for reducing spatial redundant information of a video signal, a predicted frame (called as P picture) which has a correlation between the frames through a forward prediction, and an interpolated frame (called as B picture) for reducing a correlation between the frames through a bi-directional prediction.
To playback the TP packet with a two or four times speed than normal, the MPEG decoder IC of the digital TV receiver normally skips the B picture and the P picture of the TP streams, and decodes only the I picture, thereby displaying on the screen.
Also, for much higher times speed, the MPEG decode IC of the digital TV receiver decodes by skipping the I picture of the TP packet. That is, the MPEG decoder IC can realize a forwarding speed of two times by decoding one of four I pictures rather than by decoding one of two I pictures.
As aforementioned, in order to perform the high times speed playback, the MPEG decoder IC has to decode only necessary parts of the I, B, and P pictures of the TP packet. However, the MPEG decoder IC has a limited bandwidth of the TP packet which is possible to parse, thereby having a limited operation to perform the fast forward times speed playback.
Also, in case of the rewind performance, a general MPEG decoder IC does not support the decoding. That is, the MPEG stream has decoding information which is almost given in a previous TP packet, so that a decoding is impossible in case that information of the previous packet does not exist.
To solve this problem, a software method is generally used.
That is, the MPEG IC treats a group of picture (GOP) level which is starting with the I picture as one group, thereby recognizing a recorded TP as several groups of GOP and feeding the several groups of GOP inversely.
For this method, a software program parses from an end of the stored TP packet inversely to find a starting time point of the GOP. Then, the software program feeds the MPEG decoder inversely from a GOP adjacent to the end of the TP packet end to the starting time point of the GOP.
Since the backward or the fast backward playback by the software method playbacks after parsing the stored TP packet inversely, it is difficult to realize much higher time speeds.